Buggy code:
public class HTMLAssertionGui extends AbstractAssertionGui implements ActionListener , KeyListener { private static final Logger log = LoggingManager . getLoggerForClass ( ) ; private static final long serialVersionUID = 1L ; private static final String WARNING_THRESHOLD_FIELD = "warningThresholdField" ; private static final String ERROR_THRESHOLD_FIELD = "errorThresholdField" ; private JTextField errorThresholdField = null ; private JTextField warningThresholdField = null ; private JCheckBox errorsOnly = null ; private JComboBox docTypeBox = null ; private JRadioButton htmlRadioButton = null ; private JRadioButton xhtmlRadioButton = null ; private JRadioButton xmlRadioButton = null ; private FilePanel filePanel = null ; public HTMLAssertionGui ( ) { } public String getLabelResource ( ) { } public TestElement createTestElement ( ) { } public void modifyTestElement ( TestElement inElement ) { } public void clearGui ( ) { } public void configure ( TestElement inElement ) { } private void init ( ) { setLayout ( new BorderLayout ( 0 , 10 ) ) ; setBorder ( makeBorder ( ) ) ; add ( makeTitlePanel ( ) , BorderLayout . NORTH ) ; JPanel mainPanel = new JPanel ( new BorderLayout ( ) ) ; VerticalPanel assertionPanel = new VerticalPanel ( ) ; assertionPanel . setBorder ( BorderFactory . createTitledBorder ( BorderFactory . createEtchedBorder ( ) , "Tidy<seq2seq4repair_space>Settings" ) ) ; HorizontalPanel docTypePanel = new HorizontalPanel ( ) ; docTypeBox = new JComboBox ( new Object [ ] { "omit" , "auto" , "strict" , "loose" } ) ; docTypePanel . add ( new JLabel ( "Doctype:" ) ) ; docTypePanel . add ( docTypeBox ) ; assertionPanel . add ( docTypePanel ) ; VerticalPanel formatPanel = new VerticalPanel ( ) ; formatPanel . setBorder ( BorderFactory . createTitledBorder ( BorderFactory . createEtchedBorder ( ) , "Format" ) ) ; htmlRadioButton = new JRadioButton ( "HTML" , true ) ; xhtmlRadioButton = new JRadioButton ( "XHTML" , false ) ; xmlRadioButton = new JRadioButton ( "XML" , false ) ; ButtonGroup buttonGroup = new ButtonGroup ( ) ; buttonGroup . add ( htmlRadioButton ) ; buttonGroup . add ( xhtmlRadioButton ) ; buttonGroup . add ( xmlRadioButton ) ; formatPanel . add ( htmlRadioButton ) ; formatPanel . add ( xhtmlRadioButton ) ; formatPanel . add ( xmlRadioButton ) ; assertionPanel . add ( formatPanel ) ; errorsOnly = new JCheckBox ( "Errors<seq2seq4repair_space>only" , false ) ; errorsOnly . addActionListener ( this ) ; assertionPanel . add ( errorsOnly ) ; HorizontalPanel thresholdPanel = new HorizontalPanel ( ) ; thresholdPanel . add ( new JLabel ( "Error<seq2seq4repair_space>threshold:" ) ) ; errorThresholdField = new JTextField ( "0" , 5 ) ; errorThresholdField . setName ( HTMLAssertionGui . ERROR_THRESHOLD_FIELD ) ; errorThresholdField . addKeyListener ( this ) ; thresholdPanel . add ( errorThresholdField ) ; thresholdPanel . add ( new JLabel ( "Warning<seq2seq4repair_space>threshold:" ) ) ; warningThresholdField = new JTextField ( "0" , 5 ) ; warningThresholdField . setName ( HTMLAssertionGui . WARNING_THRESHOLD_FIELD ) ; warningThresholdField . addKeyListener ( this ) ; thresholdPanel . add ( warningThresholdField ) ; assertionPanel . add ( thresholdPanel ) ; <START_BUG> filePanel = new FilePanel ( JMeterUtils . getResString ( "file_visualizer_output_file" ) , ".txt" ) ; <END_BUG> assertionPanel . add ( filePanel ) ; mainPanel . add ( assertionPanel , BorderLayout . NORTH ) ; add ( mainPanel , BorderLayout . CENTER ) ; } public void focusLost ( FocusEvent inEvent ) { } public void focusGained ( FocusEvent e ) { } public void actionPerformed ( ActionEvent e ) { } public void keyPressed ( KeyEvent e ) { } public void keyReleased ( KeyEvent e ) { } private void validateInteger ( JTextField field ) { } public void keyTyped ( KeyEvent e ) { } }

Fix:
filePanel = new FilePanel ( JMeterUtils . getResString ( "html_assertion_file" ) , ".txt" ) ;